I. Field of the Invention
This invention relates to a method of and apparatus for producing flush solder fill over recessed pins and over passage holes in circuitized substrate.
II. Description of the Prior Art
Various soldering systems are well known in the art for mass soldering electrical and electronic components, by their leads, onto circuitized substrates. Typically, such substrates are composed of a dielectric material such as ceramics or of a glass/polymer base composition. One technique for mass soldering components to substrates is that of dip soldering. With this technique, the entire side of a substrate containing the printing wiring, with the leads from the components projecting through apertures in the board, is engaged for a certain period of time with the surface of a bath of molten solder, and then removed. Another technique for mass soldering components onto substrates is that of wave soldering. A typical prior art wave soldering system generally comprises a container adapted to hold a supply of molten solder and a sump partially submerged in the molten solder. The sump has an intake orifice below the surface of molten solder, and an elongated horizontal nozzle or slot above the surface of the solder. A positive displacement pump is submerged in the body of solder and is adapted to force molten solder into the sump intake orifice, where the molten solder then flows upward in the sump and out the horizontal nozzle to thereby produce a smoothly rounded standing wave of molten solder above the nozzle.
Other techniques for mass soldering electrical and electronic components onto circuitized substrates are well known in the art and include cascade soldering, jet soldering and drag soldering. So-called "leadless" components such as flat packs can also be mass soldered to substrates by fixing the components to the bottom of a substrate, as by fixturing or with an adhesive, and then engaging the bottom of the substrate and the components with molten solder. While known mass soldering systems have provided substantial manufacturing economy to the electronics industry and thus achieved substantial commercial use, the deposition of excess solder on the substrate circuits, connections and leads has been a continual problem. Deposition of excess solder may result in formation of shorts, icicles and/or bridges, and will increase solder consumption and finished substrate weight. Moreover, current trends in the electronics industry to relatively high density electronic assemblies has increased the problem of solder shorts, icicling and bridging.
The prior art has devised various techniques to solve the problems of solder shorts, icicling and bridging. For example, for wave soldering, one technique which has become virtually universally adopted by the industry is to incline the travel path of the substrates through the solder wave, i.e. from the horizontal, to increase the exit angle between a board being soldered and the solder wave. The art has also devised various wave geometries for further increasing the exit angle and/or changing the point where a substrate exits the wave. Another system for reducing the incidence of solder shorts, icicling and bridging which has achieved substantial commercial acceptance, is to intimately mix soldering oil in the solder wave in accordance with the teachings of Walker et al, U.S. Pat. No. 3,058,441. While such systems have been found to reduce substantially the incidence of solder shorts, bridging and/or icicling, such systems have not entirely eliminated the problem, particularly in cases where relatively high density electronic assemblies and/or relatively long lead components are being soldered to substrates.
Another difficulty commonly encountered has been the inability to control the solder level over a recessed pin in the substrate. The requirement to use the space on small modules of memory chip circuitry in the most efficient manner has led to the flush pin substrate. In this arrangement, the silicon chip and the through pinned ceramic are layered in such a way that the chip lies above as well as beside the pin heads. The clearance between the bottom surface of the chip and the highest part of the metallized and soldered substrate is most important to ensure proper chip-to-substrate joining and joint reliability. To accommodate this, maximum heights of solder, pins and circuitry are specified. These specifications call for a maximum height of 0.003 inches for so-called "C4 fingers" and 0.001 inches elsewhere above the plane of the ceramic surface. Immersion tinning leaves so much solder on the surface especially, at the pin head to circuit connection, that the height specification is exceeded unless special treatment is performed. Previous expedients for counteracting or leveling the solder balls which resulted from the soldering operation have included brushing the solder while still molten and milling the solder after it cools and hardens. Both of these expedients are relatively difficult to achieve in the confined space existing on the surface of a substrate and relatively expensive because of the individualized nature of each operation.
A number of patents typify the prior art relating to leveling of solder on a substrate and removal of excess solder. U.S. Pat. No. 4,083,323 discloses the use of opposed gas knives to remove excess molten solder from a printed circuit board as the board is being withdrawn from a bath of molten solder.
According to U.S. Pat. No. 4,101,066, a circuit board is advanced along a predetermined path so that it engages, in rapid succession, a pair of solder waves flowing in opposite directions, an expedient which is said to reduce crossovers or bridges between the terminals and the conductor paths. A mechanical expedient in the form of cleaning bands provided with bristles to clean the circuit board of soldering and resin remnants arising when soldering components on the board is disclosed in U.S. Pat. No. 4,354,292. According to U.S. Pat. Nos. 4,401,253, 4,402,448, 4,410,126, and 4,469,716, a fluid stream is directed onto a circuit board immediately following deposition of molten solder onto the bard, for eliminating excess solder before it solidifies as shorts, icicles, or bridges. Hot oil is applied to the circuit board surface while solder previously applied is still molten to help alleviate the problem of solder icicles, bridging, etc. according to U.S. Pat. No. 4,463,891.
Yet other expedients, such as rapid deceleration and impact of the frame holding a circuit board to thereby dislodge excess solder, or vibrating circuit boards or other coated articles for the same purpose, are presented in U.S. Pat. Nos. 4,129,668 and 4,501,770. It has also been considered to apply a measured amount of solder to each pin head.
However, as noted above, many of these expedients require complex machinery with the high cost associated therewith, or do not readily lend themselves to high volume, mass production operations, or are not completely effective in achieving the solder leveling and remnant removal which is necessary in order to achieve an end product which is of high quality.
A recently devised method which has experienced a degree of success is disclosed in commonly assigned U.S. patent application Ser. No. 837,817 filed Mar. 10, 1987, now U.S. Pat. No. 4,676,426 issued June 30, 1987 and entitled "Solder Leveling Technique." According to that invention, a pre-fluxed substrate is dipped into a molten solder bath, withdrawn to cool to a temperature below 300.degree. F. then re-dipped into the solder bath, and again withdrawn to cool to room temperature. This procedure results in a leveling of solder balls which had formed over recessed pins and flush filling of passage holes present in the substrate. However, unlike the foregoing recent development, the instant invention utilizes existing machinery and equipment. Furthermore, while the invention results in significant improvements in the quality of the end product and efficiency and economy of its fabrication, this effect as been achieved with minimal change to existing machinery and equipment.